1. Field of the Invention
The present invention relates to a fuel cell apparatus and to a method of controlling the fuel cell apparatus.
2. Description of the Related Art
Conventionally, fuel cells, which are high in power generation efficiency and exhaust no toxic substance, have been put into practical use as a power generation apparatus for industrial use or home use or as a power source of an artificial satellite, spacecraft, or the like. Meanwhile, in recent years, developments toward use of a fuel cell as a power source of a vehicle such as automobile, bus, or truck have progressed.
Such a vehicle includes many auxiliary apparatuses, such as lights, a radio, and power windows, which consume electricity even when the vehicle is stopped, and such a vehicle travels in various patterns. Therefore, a power source used in a vehicle is required to supply sufficient power in a considerably wide range of operation conditions. Accordingly, when a fuel cell is used as a power source for a vehicle, a hybrid system which includes a battery (storage battery or secondary battery) as well as a fuel cell is generally employed.
FIG. 1 shows a conventional fuel cell apparatus.
In FIG. 1, reference numeral 101 denotes a fuel cell, which is generally a polymer electrolyte membrane fuel cell (PEMFC), but may be an alkaline fuel cell (AFC), a phosphoric acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), or a direct methanol fuel cell (DMFC).
Reference numeral 102 denotes a battery which can repeat discharge upon charging, such as a lead-acid battery, a nickel-cadmium battery, a nickel-hydrogen battery, a lithium ion battery, or a sodium sulfur battery.
Reference numeral 103 denotes an inverter (INV). The inverter 103 converts direct current output from the fuel cell 101 or the battery 102 to alternating current and supplies the alternating current to an unillustrated AC motor which serves as a drive source for rotating wheels of a vehicle. Notably, when the drive source is a DC motor, the direct current output from the fuel cell 101 or the battery 102 is supplied directly to the drive source without passing through the inverter 103.
In the fuel cell apparatus having the above-described configuration, the fuel cell 101 and the battery 102 are connected in parallel so as to supply electric power to the inverter 103. Therefore, electric power is automatically supplied from the battery 102 to the inverter 103 when the fuel cell 101 stops upon stoppage of the vehicle or when the fuel cell 101 becomes unable to supply a required power during heavy load operation, such as while traveling uphill.
Meanwhile, when the vehicle decelerates, the AC motor serving as a drive source functions as a generator so as to generate regenerative electric power. In such a case, the regenerative electric power is supplied to the battery 102, which is charged again. Further, when the terminal voltage of the battery 102 drops due to discharge, electric power generated by the fuel cell 101 is automatically supplied to the battery 102.
As described above, in the fuel cell apparatus, the battery 102 is charged at all times, and electric power is automatically supplied from the battery 102 to the inverter 103 when the fuel cell 101 becomes unable to supply a required level of power. Therefore, the vehicle can be operated stably in various traveling modes.
However, in the conventional fuel cell apparatus, since the fuel cell 101 and the battery 102 are simply connected in parallel without any control for controlling the current ratio between the fuel cell 101 and the battery 102, the current of the fuel cell 101 and the current of the battery 102 are determined by their current-voltage characteristics.
Therefore, the battery 102 always supplies electric power, and thus, the battery 102 must have a large capacity. Since the battery is generally large, heavy, and expensive, increasing the capacity of the battery 102 results in corresponding increases in volume, weight, and cost of the vehicle.
In the case in which the terminal voltages of the fuel cell 101 and the battery 102 are set so as to reduce the voltage difference therebetween, even when the terminal voltage of the battery 102 drops due to discharge, large current does not flow from the fuel cell 101 to the battery 102, with the result that charging the battery 102 requires a long time. When the terminal voltages of the fuel cell 101 and the battery 102 are set so as to increase the voltage difference therebetween, large current (electric power) flows from the fuel cell 101 to the battery 102, with the result that the battery 102 may be broken due to overcharging.
Moreover, in general, the voltage-current characteristic of a battery varies with the remaining capacity, which makes it difficult to maintain a predetermined ratio of output between the fuel cell 101 and the battery 102 to thereby allow the fuel cell 101 and the battery 102 to exhibit their original current-voltage (or electric power) characteristics. Therefore, the following problems may occur. Even when the fuel cell 101 becomes unable to supply a required power during heavy load operation, such as while traveling uphill, no electric power is supplied from the battery 102 to the inverter 103, and thus, traveling of the vehicle is restricted. Even when the remaining capacity of the battery 102 decreases, no electric power is supplied from the fuel cell 101 to the battery 102, with the result that the battery 102 becomes dead.
In order to solve the problems involved in the conventional fuel cell apparatus, the present inventor has proposed an improved fuel cell apparatus and an improved method of controlling a fuel cell apparatus (see Japanese Patent Application No. 2000-362597).
The fuel cell apparatus comprises a fuel cell, a load connected to output terminals of the fuel cell, and an electricity accumulation circuit including an electricity accumulator and connected in parallel to the fuel cell. The electricity accumulator supplies electric power to the load when electric power supplied from the fuel cell is less than electric power required by the load. The electricity accumulator is charged by regenerative electric power generated at the load and electric power generated by the fuel cell. The electricity accumulation circuit further includes a step-up circuit for increasing voltage output from the electricity accumulator and for supplying electric power to the load; a charging circuit for supplying to the electricity accumulator electric power output from the fuel cell in order to charge the electricity accumulator; and traveling condition detection means for detecting the traveling state of the vehicle. The step-up circuit and the charging circuit are selectively operated in accordance with the traveling state of the vehicle as detected by the traveling condition detection means.
The method is adapted to control a fuel cell apparatus which includes a fuel cell having terminals connected to a load; and an electricity accumulation circuit connected in parallel to the fuel cell and including a step-up circuit, a charging circuit, and an electricity accumulator, wherein the method controls electric power charged into the electricity accumulator and electric power supplied from the electricity accumulator to the load.
The fuel cell apparatus and the method of controlling a fuel cell apparatus proposed by the present inventor have solved the problems of the conventional fuel cell apparatus. Thus, it becomes possible to properly control the current (electric power) ratio between the fuel cell and the battery to thereby enable proper charging of the battery, prevent the capacity of the battery from increasing, and maintain a predetermined ratio of output between the fuel cell and the battery.
However, the fuel cell apparatus and the method of controlling a fuel cell apparatus proposed by the present inventor premise that the performance and operation of the fuel cell are stable at all times and that the fuel cell outputs electric power constantly. If a large load is imposed on the fuel cell, the temperatures of the electrolyte film and the electrodes of the fuel cell increase, and in the worst case, the electrolyte film and the electrodes burn out. Even in the case in which the electrolyte film and the electrodes do not burn out, the performance of the fuel cell deteriorates greatly, or the operation becomes unstable. Meanwhile, in order to enable the fuel cell to output electric power constantly, fuel such as hydrogen gas must be supplied to the fuel cell at a constant rate. If the flow rate of the supplied fuel gas becomes lower than the flow rate that the fuel cell requires to output a required electric power, carbon or other components contained in members that constitute the fuel cell cause reaction, with the result that the fuel cell burns out. On the other hand, when the pressure of the supplied fuel gas is excessively high, a member that constitutes the fuel cell may be broken.